remove Ubuntu-specific ondemand.service

remove Ubuntu-specific ondemand.service

New processors handle scaling/throttling in internal firmware
(e.g. intel_pstate), and do not require OS config.

Additionally, nobody else does this, not even Debian.

And finally, this has caused problems for years, e.g.:

https://bugs.launchpad.net/ubuntu/+source/sysvinit/+bug/1497375
https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/1503773
https://bugs.launchpad.net/ubuntu/+source/sysvinit/+bug/1480320
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1579278
https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/1806012
https://bugs.launchpad.net/charm-sysconfig/+bug/1873028